1. Field of the Invention
This invention generally relates to a spectral-selective solar energy absorber or collector and a process for producing the same, and more particularly to a process for easily and economically forming with hgh reproducibility a spectral-selective coating or film on a surface of an aluminum substrate or base material so as to efficiently utilize the solar energy.
2. Description of the Prior Art
With the recent oil crisis as a turning point, an aggravating exhaustion of energy resources, particularly fuel oil, has been increasingly a serious problem, which has necessitated and accelerated the pace of various efforts and research directed toward maximum utilization of all energy resources available in many different forms. Among the presently available energy resources, solar energy or heat has been and is considered to be one of the most easily or readily usable energy sources available anywhere on the earth, and thus there is demand to rapidly develop and establish technologies for effective utilization of the solar energy. The solar energy is generally obtained in the form of heat usually absorbed by a black or dark-colored surface of a body, for example, a surface coated with a thin film of a black paint. While such a conventional heat absorbing surface or coating is efficient in terms of selective absorption of solar energy, its high emissivity nature creates a problem of energy loss, i.e., the solar energy or heat absorbed by such a surface tends to be emitted or radiated as infrared rays. This results in the failure in effectively conducting the absorbed solar heat to a suitable medium such as a water, that is the temperature of the water can not be elevated in a sufficiently effective manner by transfer of the heat from the absorbing surface to the water. Thus, the conventional solar energy absorbing surface with a black paint coating per se is limited in its application and therefore suffers a potential disadvantage of being incapable of efficiently utilizing the solar energy.
To solve the above indicated problem and disadvantage, it is an important requirement to develop a spectral-selective solar energy absorbing surface or absorber which is characterized by its high absorptance of the solar energy, particularly in the visible and near-infrared regions which are a major portion of the solar spectrum, as well as by its low emissivity in the long wavelength (infrared) region, the said selective absorber having, in other words, optical properties which assure efficient absorption of the solar light (high solar absorptance nature) and less thermal emission from the absorbing surface (low thermal emissivity nature).
As a result of extensive study and research of a selective coating or film, made under such situation, the inventors of the present invention and coworkers had found novel processes usable for forming, on a substrate of an aluminum material, black coatings which are capable of efficient selective absorption of the solar energy, for which Japanese patent applications were filed as TOKU-GAN-SHO 53-62480 and 54-15900 (respectively published as TOKU-KAI-SHO 54-153348 and 55-107856). The process which is the subject matter of the latter application, comprises a step of subjecting a smooth surface of a magnesium (Mg)-containing aluminum (Al) alloy material to a chemical conversion treatment to form thereon a chemical conversion coating containing chromium (Cr), and a subsequent step of subjecting the Al alloy material to an oxidation treatment at a temperature of 400.degree.-600.degree. C. in order to finally form an oxidized coating on the surface of the alloy, thereby making it possible to easily and economically forming an oxidized, dark-colored coating that demonstrates excellent properties of solar absorptance and specular reflectance, and provides an increased efficiency in selective absorption of the solar energy.
While the above described process has such excellent features, the inventor of the present invention who made further study and research on such oxidized, dark-colored coatings for more improvement of the process for forming the same under various conditions, found that the surface of the Al alloy material subjected to the chemical conversion treatment could not always be sufficiently colored, i.e., not blackened as intended, through the final oxidation treatment at a high temperature. Thus, it became clear that the previously indicated process had such disadvantage that would reduce the benefits arising out of its advantage of being high in reproducibility. To adapt the above process to use for industrial production, accordingly, it has been desired and is preferred to employ a suitable method for overcoming the inherent problem of insufficient coloration during the high-temperature oxidation treatment of the process and thereby forming with increased reproducibility well blackened or sufficiently dark-colored, oxidized coatings on the surface of the aluminum alloy substrate.